Method and apparatus for electrodeposition of a layer of uniform thickness on a conductive surface



June 12. 1956 R. A. MILLER 2,750,332

METHOD AND APPARATUS FOR ELECTRO-DEPOSITION OF A LAYER OF UNIFORM THICKNESS ON A CONDUCTIVE SURFACE Filed June 4, 1952 IN V EN TOR.

BY j?0eri 17. Miller aw if arrow/5v United States Patent .METHGD AND APPARATUS FOR ELECTRO- DEPOSITION F A'LAYER OF UNIFORM THICK- NESS ON A CONDUCTIVE SURFACE .Robert A. Miller, Tarentum, Pa assignor to Pittsburgh Plate Glass Company, Allegheny County, Pa,, acorporation of Pennsylvania Application June 4, 1952,-Serial No. 291,609

6 Claims. (Cl. 204-15) "This "invention relates to a method and apparatus for the electro-deposition of a metallic layer of constant'thick- -fi1m=results from the relatively high electrical resistance of the thinprimary film.

For example, one prior method of applying --the outer copper film has been to hold the mirrored I glass plate stationary in a bath of copper sulphate with copper anode bars suspended in the solution .approximately 4 inches from the silvered surface. The silver 'film twas 'connectedto theopposite pole of the system by means 'of suitableelectrical-contacts to constitute the cathode. The

Non-uniformity in the thickness of the deposited outer.

bath-was-usually a-highly acid saturated solutioniof copper sulphate; thecurrent densityapproximately 3. amperes per square feet at 155 volts, and the plating time was approximately 3 minutes. The copper coating varied'from a heavy layer at the edges-of the mirror where the clamps were attached, about the periphery, to essentially :no

sensible thickness at the center, remote from the con-' tacts, on a reasonably large mirror. At :the edges 0f the plate, the weight of copper-might run ashigh. asf0.50

gram per square .f0ot.or a thickness vof 0.000024 inch, whereas, at rthe :center the .weight 'Of copper may approach 0.08 gram .per square foot, or :a thickness of 10.000004 inch, or less.

'A principal object of this :invention, therefore, :is :to

:providea method and apparatus by whichan outer 3,01 :protective metal film of uniform thickness may b6 applied to :a ithinprimary film on a large surface .TSuCh as zthecsilver filrn on a mirror.

A certain degree .ofuniformity of rate :of .deposition .of :the secondary film .may be achieved by :providing a plating bath vessel of conical or pyramidal shapegin which the anode is positioned at the apex ofthe cone ortpylamid -with:the primary film as the cathode at the base of such cone 1 or pyramid. The shape of t the plating-bath would .have :to be carefully. calculated and might have .to I be reproportioned for primary films .of different :sizes and :shapes.

.Another method of obtaininguniform'depositiontislto dispose electrical contacts at a :plurality of points on the'primary film but such a method would not .be

desirable for use in .the case of mirror films .forthe contacts would damage the primary silver film .and .defeat' v.the purpose .of applying .a protective .coating to .the pri- .mary film.

The method and apparatus of this .inventionelimr 'natesthe immersion of the glass andprimaryfilm. In-

stead, the glass 'with the electrically conductive silver ice .film is supported and conveyed past an ,electro-deposition station. The electrolyte solution is contained in a supply tank at the station. A positive electrode is .disposed in the tank and immersed in the solution. A nozzle .outlet .or applying means is connected to the-tank andis arrangedgto feed astream of electrolyte onto the primary film across the width of the film. The negative electrical contacts are made along the edge of .the primary -film whichsis -topass through the deposition station first.

In operation, with the positive and negative electrodes s uit-ablyponnected to a source of power, :the stream of electrolyte is fed first onto the primary :-film at ,the edge where the cathode contacts are placed. The .electric -,curgrent between the anode metal in the .electrolyte'tank and .the cathode contacts ,on :the primary .film .is :maintained :by -;the .continuous flow ,of :the electrolyte .onto the primary film. As a result, the anode metal is deposited on the :primary film along the edge near the contacts .on the primary film. The movement of .the glass and the iprimariy :film {is such thatthe desired thickness is apmlied first along that edge, and then as the glass andthe primary-film is moved at a uniform speed the-edgeofthe primary .filnnalong which the cathode contacts are made moves away from the electrolyte feeding means =in the deposition station. As the .glass and primary ifilm move through :the :station, successive .areas .of :the primary film twillthusrbe covered with electrolytezandtthe outeranetal will "be :deposited in suchareas. .A receiving :tank -be'low the .deposition station serves to collect the spent solu- 'ztion.

For a more .complete description of theinvention, referenceis made to the drawings, wherein:

Pig. 1 is .a schematic side view of one form of apparatusembodying the invention;

Fig. 21s a view partly in section, taken along the line 2.r 2'in Fig. 1;

Fig. 3 is a perspective view of a glass plateshowing :the relationship of the primary and the outer-films.

IFigHJ L-is a'longiutdinal cross sectional viewof analten --'l13iI6-'fOIII1 of electrolyte feeding nozzle; I

Fig. 5 .is a bottom plan view of the alternate tformof nozzle shown in Fig. 4. V

- The apparatus illustrated in the drawings includes a conveyor such as a roller conveyor 10, illustrated schematically in Fig. 1, having rollers 1-1=for supporting a glass-plate 12. The plate l'l-hasaprimaryfilm of silver or-other suitable material 13 upon one face. *The glass is'orientedwith the silver film uppermost.

A supply tank or container 15 suitable for receiving anelectrolytic solution 16 may bepositioned,-above=the glass, in any suitable manner. A nozzle 1'7 is provided -which=is connectedto the tank 15 by suitable means-such as connecting hose 18.

The nozzle 17 may be of asize to extend across the width-of the glass mirror being'treated. The lower :side of the nozzle may be formed to define a plurality of small diameter apertures 20 through which electrolyte -may be fed-onto the primary film 13.

At least one anode or positive electrode 21 of :the -metal to be plated maybe immersed inthesupplyfiank -15.

'A-primaryfilm 13 forms the cathode or.-negativeelectrode, electrical contact to the film'beingma'debyasuitable .means such as one'or more alligator clips;22 .alongtone rend of the .glass.

of..t l1e.;outer film metal has beendeposited ,on nthe pri- .mary filmin the areaaround the clips. Thereafter, .the ,glass 12 is moved, as indicatedby the arrowjn 1,

from right to'left, so that there'is a uniform deposition prevent injury to such film but effectively confine it of the outer film at progressively remote areas from the cathode contacts at the end of the primary film. A re-- ceiving tank 23 may be provided below the glass plate 12 in the general area of the nozzle 17 to recover the used electrolyte solution. A suitable drain 25 may be provided for drawing the solution from the receiving tank.

Electric current flows through the apparatus only when the stream of electrolyte is flowing upon the primary film. The thickness of the secondary film will depend 'upon the current density at the electrolyte nozzle 17,

upon the rate at which the primary film is moved past the nozzle, also upon the concentration of cations in the electrolyte and upon the temperature.

As a guide to the variables effecting the thickness of the deposition of the outer film, it has been found that various copper sulphate solutions may be used ranging from one of high acidity such as pH 1.5, to one of high alkalinity such as pH 8 or 9. More rapid deposition is obtainable from reasonably high acidity than'from either neutral or alkaline solutions, whereas a brighter looking copper film is obtained from a neutral or slightly alkaline solution. The copper sulphate bath should be a nearly saturated solution. The pH value in the solution can be controlled by the addition of H230; to lower the pH or the addition of a suitable buffer solution to raise the pH.

The current density may vary from approximately 1.5

amperes to or 6 amperes per square foot at a voltage of from 1.5 to 5, the area to be considered being that of the solution-dispensing nozzle, carrying the current and the copper solution to the primary film. The desired current density and voltage may be readily determined.

As an example, at a current density of 3 amperes.

over a nozzle area of 12 square inches, with a voltage of 1,5, and a forward motion of the plate at a rate of 12 inches per minute, the copper film thickness resulting is approximately 0.000008, or a weight of .178 gram of copper per square foot. The amount of copper deposited will vary directly with the amperage and voltage and inversely with the speed of forward movement of the plate being coated.

The temperature of the solution should normally be maintained at about 70 to 75 F. If the solution is colder, the amount of copper deposited will be less, although it will be more firmly bonded and brighter. If the solution is warmer, the amount of copper deposited will be greater, however, it will be less firmly bonded and more porous.

In the apparatus illustrated, the nozzle 17 shown in Fig. 2 isarranged to extend across the entire width of .the mirror and the outlet means may be 72" in length to span the entire width of a large mirror. It will be appreciated that conventional overhead supports (not shown) along the length of the nozzle may be provided to support it. The nozzle is spaced very close to the surface of the primary film but the supports for the nozzle and the glass should be such that the nozzle does not come in contact with the primary film at any time. 7

Figs. 4 and 5 illustrate an alternate form of nozzle 35 having a single slit aperture 36 and having felt members 37 on both sides of the aperture 36 to confine the electrolyte solution 38 being fed from the nozzle 35 onto the primary film 40 on the glass 41.

are maintained out of contact with the primary film to the electrolyte to the plating area.

Although for simplicity of showing the cathode or negative contacts are illustrated as clips 22in contact with the upper surface of the primary film 13 and also in contact with the deposited outer film 14, in practice, it

It will be noted from Fig. 4 that the lower faces of the felt members 37 be a line contact 24 in electrical engagement with the the most uniform and economical plating with the highest deposition of copper, the line contact should remain in fixed position on the edge" of the mirror and move forward with it.

It will be appreciated from the foregoing that the method and apparatus of this invention fulfills the objects of applying a thin outer film of uniform thickness to a thin primary film.

In accordance with the provisions of the patent statutes, I have herein described the principle of operation of the invention, together with the elements which I now consider the best embodiments thereof, but I desire to have it understood that the structure disclosed is only illustrative and the invention can be carried out by other means. For example, whereas, in the illustrated embodiment of the invention the glass containing the mirror to be coated is moved while the nozzle means for feeding the electrolyte is stationary, it will be appreciated that a moving nozzle with respect to a stationary mirror is also contemplated within the scope of the invention. Also, while it is designed to use the various features and elements in the combinations and relations described, some of these may be altered and modified without interfering with the more general results outlined.

Having thus described my invention, I claim:

1. In apparatus for the electro-deposition of a metallic layer of constant thickness upon an electrically conductive surface having a conveyor adapted for supporting and conveying the said surface along a predetermined path and past an electro-depositing station for presenting sucessive areas of the conductive surface at said station, a supply tank containing an electrolytic bath, a positive electrode disposed in the supply tank and immersed in the bath, an applicator connected to said supply tank, a negative electrode connected to the electrically conductive surface and with the electrolyte on said surface, and

tive surface having a conveyor adapted for supporting and conveying the said surface along a predetermined path and past an electro-depositing station for presenting successive areas of the conductive surface at said station, a'supply tank containing an electrolytic bath, 'a positive electrode disposed on said surface, and means to connect a source of electricity to the positive and negative electrodes, said applicator comprising an elongated nozzle extending over the full width of said station transversely to said predetermined path and having a slit centrally located therein for projecting said electrolyte onto the conductive surface at said statiomsaid nozzle also including depending absorbent material means attached to and extending continuously around-the edges of said nozzle, said applicator being mounted at said station and relative to said conveyorso that said depending means substantially confines the electrolyte to said station without contacting said conductive surface.

3. In apparatus for the electro-deposition of a metallic layer of. constant thickness upon an electrically conductive surface having a conveyor adapted for supporting and conveying the said surface along a predetermined primary film across the entire width of the mirror. For path and past an electro-depositing station for presenting successive areas of the conductive surface at said station, a supply tank containing an electrolytic bath, a positive electrode disposed in the supply tank and immersed in the bath, an applicator connected to said supply tank, a negative electrode connected to the electrically conductive surface and with the electrolyte on said surface, and means to connect a source of electricity to the positive and negative electrodes, said applicator comprising an elongated nozzle extending over the full width of said station transversely to said predetermined path and having a slit centrally located therein for projecting said electrolyte onto the conductive surface at said station, said nozzle also including depending absorbent material means attached to and extending continuously around the edges of said nozzle for substantially confining the electrolyte to said station, said applicator being mounted at said station and relative to said conveyor so that said depending means substantially confines the electrolyte to said station without contacting said conductive surface.

4. In apparatus for the electro-deposition of a metallic layer of constant thickness upon an electrically conductive surface having a conveyor adapted for supporting and conveying the said surface along a predetermined path and past an electro-depositing station for presenting successive areas of the conductive surface at said station, a supply tank containing an electrolytic bath, a positive electrode disposed in the supply tank and immersed in the bath, an applicator connected to said supply tank, a negative electrode connected to the electrically conductive surface and with the electrolyte on said surface,

and means to connect a source of electricity to the positive and negative electrodes, said applicator being mounted at said station and. comprising an elongated nozzle extending over the full width of said station transversely to said predetermined path and having a slit centrally located therein for projecting said electrolyte onto the conductive surface at said station, an absorbent material edge depending from the nozzle and surrounding said slit, said edge extending close to the conductive surface for confining said electrolyte to said station but maintaining a clearance to avoid contact with said conductive surface.

5. The invention according to claim 4 wherein said absorbent material edge is relatively thick to provide good absorbing capacity.

6. The invention according to claim 5 wherein said absorbent material edge is constructed of felt.

References Cited in the file of this patent UNITED STATES PATENTS 883,756 Steiner Apr. 7, 1908 1,416,929 Bailey May 23, 1922 1,582,407 King Apr. 27, 1926 1,603,951 Hitchcock Oct. 19, 1926 1,771,680 Ishisaka July 29, 1930 1,809,826 Bohlman June 16, 1931 2,540,602 Thomas et al. Feb. 6, 1951 FOREIGN PATENTS 16,852 Great Britain of 1896 287,402 Great Britain Mar. 22, 1928 39,808 Sweden Dec. 15, 1915 

1. IN APPARATUS FOR THE ELECTRO-DEPOSITION OF A METALLIC LAYER OF CONSTANT THICKNESS UPON AN ELECTRICALLY CONDUCTIVE SURFACE HAVING CONVEYOR ADAPTED FOR SUPPORTING AND CONVEYING THE SAID SURFACE ALONG A PREDETERMINED PATH AND PAST AN ELECTRO-DEPOSITING STATION FOR PRESENTING SUCESSIVE AREAS OF THE CONDUCTIVE SURFACE AT SAID STATION, A SUPPLY TANK CONTAINING AN ELECTROLYTIC BATH, A POSITIVE ELECTRODE DISPOSED IN THE SUPPLY TANK AND IMMERSED IN THE BATH, AN APPLICATOR CONNECTED TO SAID SUPPLY TANK, A NEGATIVE ELECTRODE CONNECTED TO THE ELECTRICALLY CONDUCTIVE SURFACE AND WITH THE ELECTROLYTE ON SAID SURFACE, AND MEANS TO CONNECT A SOURCE OF ELECTRICITY TO THE POSITIVE AND NEGATIVE ELECTRODES, SAID APPLICATOR COMPRISING A NOZZLE FOR PROJECTING SAID ELECTROLYTE ONTO THE CONDUCTIVE SURFACE AT SAID STATION, AND INCLUDING DEPENDING MEANS ATTACHED TO THE EDGES OF SAID NOZZLE, SAID APPLICATOR BEING MOUNTED AT SAID STATION AND RELATIVE TO SAID CONVEYOR SO THAT SAID DEPENDING MEANS SUBSTANTIALLY CONFINES THE ELECTROLYTE TO SAID STATION WITHOUT CONTACTING SAID CONDUCTIVE SURFACE. 